Current methodology for determining the accuracy of a tester involves time consuming and manually intensive oscilloscope measurements. Alternatively, calibration verification programs have been written, but they are of limited use since they must be customized for each tester and modified for each part-number.
An important consideration for any tester is its ability to reproduce measurements over long periods of time. This requires calibrating a tester prior to its use and recalibrating it on an ongoing basis. This procedure is particularly advantageous for aligning signals generated at various tester channels and inputted to a Device under Test (DUT). Failure to do so has a negative impact on subsequent measurements, in particular to those pertaining to AC testing, wherein the relative positioning of signals with respect to each other is of prime importance. Moreover, signals generated at an input channel routinely require their being compared with signals outputted by the DUT. Uncalibrated channels, however, may distort this comparison and give imprecise measurements.
Environmental factors, such as variations in temperature, humidity, etc., cause measurements to drift, another source of inaccuracy, even after having completed the calibration of a tester. Measurements taken between long time intervals can be distorted and invalidate test data gathered during that time. It is therefore essential to provide a way for periodically resetting the tester to compensate for drift, even if remaining within performance specifications. A standard method for tracking a tester subject to drift is by using a "golden card", specifically designed for this purpose and customized for a given tester. Those skilled in the art will readily appreciate that such a technique is restrictive by nature and lacks the important characteristic of portability between test systems.
AC testing routinely requires characterizing turn-on (t.sub.on) and turn-off (t.sub.off) delays, or any combination thereof. More particularly, for signals outputted from a DUT, the slope of the rise time or fall time of a signal, hereinafter referred to as slew rate, assumes great importance, since tester comparators are sensitive to slew rates which, in turn, impact the precision of delay measurements.
During the process of conducting manufacturing tests, or while characterizing IC chips and the like, it is important to correlate results from one test system to those of another. Practical considerations oftentimes require that part numbers of a same technology be tested on more than one test system. Thus, correlation between one test system and another is an essential requirement. Measurements originating from a variety of testers usually bear the imprint of the tester and affect the accuracy of the measurements.
Practitioners of the art will readily appreciate that the process of calibrating testers would be greatly enhanced if comparisons between signals could be made tester independent. Comparisons, even under normal circumstances, are difficult at best and oftentimes meaningless, since each tester has its own personality. If tester independent readings could be achieved, portability from one tester to the next would ensue, a distinct and substantial advantage over the present state of the art. The process of calibrating is further complicated when testing a semiconductor wafer in which the proximity of I/O terminal pads precludes actual probing.
Standard techniques for aligning signals normally require the use of TDR (Time Domain Reflectometry), which consists of sending a signal through an open ended medium, e.g., a coaxial cable, obtaining the reflected signal and measuring the time difference, normally twice the delay path. This technique is taught by Lee in U.S. Pat. No. 4,866,685, wherein testing a board is achieved by transmitting a test signal to the board via a transmission line and analyzing each response signal returned from the board in response to the original test signal.
The path delay obtained when using TDR depends on the slew rate of a driver. (A driver is normally connected to an output terminal of a chip and normally reshapes the waveform of the DUT output signal.) During calibration, the path delay from the DUT output to the comparator of the tester is determined by using this method. This technique works properly provided the slew rate of the TDR signal and the DUT output driver signals are identical. However, they oftentimes differ considerably from each other, which may cause a significant error in the measurement. A second basic problem with TDR also exists, in that signals with fast rise time propagating through a board have their rise time significantly altered, thereby giving inaccurate and erroneous readings. Thus, the delay obtained by TDR may be inaccurate due to differences in driver slew rates between the TDR and the DUT.